Do I need to book activities in advance for Thailand?

We're going on a family holiday (2 adults, 2 teens aged 19 and 16) to Thailand in January for two weeks. We're travelling to Phuket for 10 days then Bangkok for 3. Is it necessary to book in advance the activities we want to do when we're there, such as Fantasea and elephant treking, etc? Or will we be fine to turn up and book on the day or a few days in advance when we get there?